The present invention relates to a reference electrode and to a combined electrode comprising a reference electrode and a measuring electrode.
A combined electrode contains both a reference electrode and a measuring electrode in one unitary measuring member. Many applications utilize the reference electrode and the measuring electrode in one unit because this is compact and convenient. Nevertheless a separate reference and measuring electrode can be used, and the present invention relates to improvements in a reference electrode regardless of whether this is separate or part of a combined electrode.
Reference electrodes are used together with measuring electrodes in electrochemical systems to determine the concentration of ions in a sample, e.g. the pH or pX where X represents an ion. The electrochemical potential of the reference electrode should remain as constant as possible throughout the measuring process while the potential of the measuring electrode, which can be ion-selective, is a function of the concentration of the ion being tested in the sample. The potential of the reference electrode is kept substantially constant due to the presence of a saturated electrolyte salt bridge within the cell. The potential difference between the reference electrode and the measuring electrode is indicative of the concentration of the ion and may be displayed on a millivolt instrument such as a potentiometer.
The potential of the complete electrochemical cell made up of the measuring electrode and the reference electrode can be represented by the following equation:ECell=Emeas+Eref+Ej where Emeas is the potential of the measuring electrode, Eref is the potential of the reference electrode and Ej is a junction potential.
The junction potential arises from the different rates of mobility of anions and cations at the interface between two electrolytic solutions. Ideally, the junction potential of the reference electrode should have a negligible variation between solutions as otherwise the change in the junction potential will appear as an error in the overall potential calculated for the sample and hence the concentration determined.
Thus, where pH is being measured, a reference electrode ideally produces a constant millivolt potential in all aqueous liquids regardless of whether this be pure water, oily water, contaminated water or otherwise. A measuring electrode produces a different potential depending on the pH of the solution. Thus, using both electrodes together allows an accurate determination of the pH of a solution, taking into account a baseline measurement.
Well-known reference electrodes include the Ag/AgCl type and the calomel type amongst others; any convenient half-cell of electrode and electrolyte can be used in reference systems.
Some conventional reference electrodes require the electrolyte to leak slowly through an opening in the reference electrode, for example at a porous plug (e.g. made from wood or teflon or other porous materials), to form a liquid junction between the salt bridge and the sample solution being tested. In certain applications, a precipitate can form at the junction of the reference electrode and the sample solution where the electrolyte of the reference electrode flows into the sample. This precipitate can clog the opening and therefore interfere with the liquid junction potential and give rise to measuring errors. Furthermore, porous electrodes are prone to fouling, poisoning or back-diffusion of medium, and also lack robustness in the presence of temperature and pressure variations.
The problems associated with porous reference electrodes have previously been addressed by using a salt-loaded resin to surround the electrode chamber so as to provide a barrier to liquid movement whilst ensuring electrochemical communication. Such a system is described in for example WO 93/15393 of Amagruss Limited and Russell. Thus, the problems of porous reference electrodes can in part be avoided by using said salt-loaded resins instead of porous members. Such ionically-conducting salt-loaded resins cannot be poisoned because there is no liquid contact and furthermore the materials are steam sterilizable at 130 degrees C. and able to withstand pressures above 30 bar.
Nevertheless, further improvements of measurement accuracy and reliability are desirable.